1. Field of the Invention
The present invention relates to a coin discriminating apparatus wherein eddy current is induced in a coin with the use of a plurality of eddy current induction coils and based on the impedances of the individual coils that vary depending on the induced eddy current, a stamp (strike) pattern on the coin surface, the material (composition) of the coin, etc. are checked to determine the denomination and authenticity of the coin. More particularly, the invention relates to a coin discriminating apparatus wherein information on surface displacement irregularities of the stamp pattern on a surface of a coin is acquired based on the impedances of the individual eddy current induction coils excited at high frequency, and information on the material of the coin is acquired based on the impedances of specified ones of the eddy current induction coils excited at low frequency.
2. Description of the Related Art
In vending machines, automatic teller machines (ATMs), etc., a coin discriminating apparatus is incorporated to determine the denomination and authenticity of coins, as a process preliminary to the calculation of the sum of inserted coins. This type of coin discriminating apparatus is generally designed to measure the outer diameter, thickness and weight of a coin and compare the measured values with previously obtained outer diameter, thickness and weight of a true coin (one of coins of different denominations to be handled) to determine the denomination of the coin, and nonauthentic coins are rejected. Among a diversity of coins, however, there are coins, for example, coins used in foreign countries, that should not be accepted but yet have features (outer diameter, thickness, weight) resembling those of true coins to be accepted, and such coins can be erroneously judged to be authentic.
Attempts have therefore been made to acquire, in the form of an image, information on surface displacement irregularities of the stamp pattern on a coin surface and recognize features of the image, thereby to determine the denomination of the coin. Because of dust or smudge adhering to a coin surface, however, it is sometimes difficult to detect with accuracy the features of the stamp pattern on the coin surface. Further, in cases where the features of an image showing the stamp pattern on a surface of a coin to be discriminated are compared with those of an image showing the stamp pattern of a true coin, it is necessary that a matching process be performed following the rotation of the image for comparison, or that an extra process such as Fourier transform be performed as needed. Thus, a complicated process is required for the discrimination of coins and the process consumes much time.
The present invention has been created to solve the above problem and is based on the knowledge that, when a electromagnetic field is applied to a coin by using an eddy current induction coil, eddy current induced in the coin by the electromagnetic field varies depending on the material, thickness, etc. of the coin and that the impedance of the eddy current induction coil varies under the influence of the induced eddy current.
An object of the present invention is to provide a coin discriminating apparatus capable of easily determining the denomination and authenticity of a coin with high accuracy. Eddy current induction coils are successively excited to apply high-frequency electromagnetic field to the coin over an entire surface thereof, and impedances of the eddy current induction coils, which vary under the influence of eddy current induced in the coin by the high-frequency electromagnetic field, are measured to obtain information on surface displacement irregularities of a stamp pattern on the coin surface. Also, low-frequency electromagnetic field is applied to the coin by using the eddy current induction coils, and impedances of the individual coils, which vary under the influence of the eddy current induced in the coin by the low-frequency electromagnetic field, are measured to obtain information on the material of the coin. Discrimination of the coin is made based on the information thus obtained.
In the coin discriminating apparatus according to the present invention, a plurality of eddy current induction coils arranged two-dimensionally so as to face a surface of a coin are excited at high frequency and at low frequency, and in synchronism with the individual excitations of the coils, the impedances of the coils, which vary under the influence of the eddy current induced in the coin, are detected. The impedances of the eddy current induction coils detected when the coils are excited at low frequency are compared with an impedance obtained beforehand using a true coin, to determine the material of the coin. Also, the impedances of the eddy current induction coils detected when the coils are excited at high frequency are acquired as feature information indicative of information on surface displacement irregularities of the stamp pattern on the coin surface, and the feature information is compared with true coin feature information obtained beforehand, to determine the denomination of the coin. Especially, a histogram showing impedance distribution is created as feature information indicative of the information on surface displacement irregularities of the stamp pattern on the coin surface, and is compared with a histogram of a true coin obtained beforehand, to determine the denomination of the coin.
In a preferred embodiment of the present invention, the eddy current induction coils are arranged on a plane in the form rectangular grid or other suitable geometry to constitute a coil array, high-frequency exciting means successively excites all the eddy current induction coils constituting the coil array at high frequency to scan the entire surface of the coin, and low-frequency exciting means excites only specified ones of the eddy current induction coils in the coil array at low frequency. The specified eddy current induction coils in particular, which are excited at low frequency, comprise a predetermined number of eddy current induction coils located approximately in a central portion of the array constituted by the eddy current induction coils.
Alternatively, the specified eddy current induction coils excited at low frequency may be constituted by a special eddy current induction coil which is separate from the aforementioned coil array and is arranged side by side or face to face with respect to the coil array.
The coin discriminating apparatus of the present invention may further comprise coin diameter measuring means for measuring the diameter of the coin based on the impedances of the eddy current induction coils detected when the coils are excited at high frequency. The apparatus may also comprise coin thickness measuring means for measuring the thickness of the coin based on the impedances of the eddy current induction coils detected when the coils are excited at high frequency.
Further, the coin discriminating apparatus of the present invention may additionally comprise image processing means for acquiring the information on surface displacement irregularities of the stamp pattern on the surface of the coin as a two- or three-dimensional image, based on the impedances of the eddy current induction coils detected when the coils are excited at high frequency.
Also, in order to induce eddy current in the surface region of the coin, the high-frequency exciting means excites the eddy current induction coils at a frequency of about 1 MHz, for example, to produce a high-frequency electromagnetic field, and in order to induce eddy current in the inner part of the coin, the low-frequency exciting means excites the eddy current induction coils at a frequency of about 100 kHz, for example, to produce a low-frequency electromagnetic field. Specifically, the high-frequency exciting means and the low-frequency exciting means are constituted by a voltage-controlled oscillator whose oscillation frequency is variably controlled by a control voltage externally applied thereto. The control voltage is varied to switch the frequency for exciting the eddy current induction coils, thereby making the voltage-controlled oscillator function as the high-frequency exciting means or the low-frequency exciting means.
Further, according to the present invention, the eddy current induction coils are selectively oscillated by being applied with the output from the voltage-controlled oscillator via a multiplexer, and the multiplexer scans the eddy current induction coils for oscillation at high speed.
The eddy current induction coils excited at low frequency may be separate from those excited at high frequency. Also, to determine the material of the coin by exciting the eddy current induction coils at low frequency, preferably, the low-frequency exciting means selectively excites the eddy current induction coils at a plurality of different frequencies over a possible range of 100 kHz to 700 kHz or continuously varies the excitation frequency for the coils within a frequency range over a possible range of 100 kHz to 700 kHz, to produce a low-frequency electromagnetic field to be applied to a plurality of portions of the coin.